The sequence of triangular numbers

1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 66, 78, 91, 105, 120, 136, 153, 171, 190, 210, 231, 253, 276, …

**631** is a prime number.

**10631** is a prime number.

**55453628211510631** is a prime number.

**786655453628211510631** is a prime number.

**10591786655453628211510631** is a prime number.

What comes next?

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The next example is the concatenation of the triangular numbers from 1 to the 1201, the resulting prime has 6560 digits. The prime starts

721801720600…..510631

The triangular number with index 1201 is 721801

If anyone wants to see the prime its available here https://www.dropbox.com/s/761r6flmg54wwum/1201%20prime.txt?dl=0

Paul.